The present invention relates to a power source for developing electrodes for electrophotographic copying machines.
In prior art electrophotographic copying machines, particularly dry-type copying machines which employ cascade development systems, representation of lines has been found good because of what is called the edge effect, whereas representation of photographs and halftones has been limited. A developing method with developing electrodes has been employed in an effort to minimize this problem.
Developing with developing electrodes, however, has various technical difficulties. A typical one is to entirely and uniformly maintain a spacing distance between a developing electrode plate and a photosensitive body; another difficulty is that of maintaining a balance of three potentials, i.e., a developing electrode potential, an image potential of the photosensitive body, and a potential in a non-image portion of the photosensitive body (background potential).
The present invention is particularly directed to an improvement in the latter.
In conventional developing electrode developing methods for electrophotographic copying machines using a photosensitive body (hereinafter referred to as a photosensitive material relying on temperature), wherein the image potential of the photosensitive body and the background potential vary due to variation of ambient temperatures, it has been difficult to acquire balance of the above-mentioned three potentials even by applying constant charging electric current and exposure to the photosensitive body, which results in a difficulty in maintaining good quality copies. Particularly, early in the morning during the winter season, ordinary offices are not sufficiently heated, and the temperature often drops to a level below 5.degree. C, as a consequence of which both the image potential and the background potential are greatly varied. Especially under the environment of low temperatures, the background potential rises to produce more foggy copies, resulting in a great displacement from the initial purpose of the developing electrode method employed to improve the copy quality.
In order to eliminate the disadvantages noted above, a prior art technology has been proposed which can always produce copies of good quality even if the ambient temperature should be varied. According to such proposals, a temperature sensor such as a thermistor or a thermocouple may be used to directly or indirectly detect a temperature of a photosensitive body or to detect a surface potential of the photosensitive body to activate a control circuit by a detection signal resulting therefrom; this sets a developing electrode voltage to an optimum value, thus always producing copies of good quality.
Such prior art devices, however, pose a drawback such that generally, the photosensitive material gradually deteriorates as the use thereof increases. This requires that the above-mentioned control circuit be corrected, particularly, where the temperature of the photosensitive body is detected, since it takes time to change a set value.